Active matrix organic light emitting diode (AMOLED) display is one of the hot researches in the field of flat panel display. Compared with the liquid crystal display, organic light emitting diode (OLED) has the advantages of low energy consumption, low production cost, self luminescence, wide viewing angle and quick response, etc. At present, in the display field of mobile phone, PDA (Personal Digital Assistant, or palmtop computer) and digital camera, OLED has begun to replace the traditional LCD display screen. The pixel driving circuit design is the core technical content of AMOLED display, which has important research significance.
Different with TFT-LCD (Thin Film Transistor Liquid Crystal Display) applying stable voltage for the control of brightness, OLED is driven by current, and stable current is needed to control the emitting of light. For the reasons such as technology process and aging of device, in the existing driving circuit with two transistor T1, T2 and one storage capacitor C1 (as shown in FIG. 1), the driving current IOLED is a current generated by applying the voltage Vdata provided by the data line on the saturation region of the driving transistor (DTFT). This current drives the OLED for emitting light, wherein the calculation formula of the driving current is IOLED=K(VGS−Vth)2, the VGS is the voltage between the gate and the source of the driving transistor, and the Vth is the threshold voltage of the driving transistor. For the reasons such as technology process and aging of device, the threshold voltages (Vth) of the driving TFTs of individual pixels are uneven. Since the threshold voltages of the driving TFTs (i.e., T2 in FIG. 1) of individual pixels are uneven, there is difference between the currents flowing through the OLEDs of individual pixels, affecting the display effect of the entire image.